A Post reporter spotted brown water from a rusty beam creeping down the wall of the front entrance of Citi Field’s main gate in Flushing, Queens, on 126th Street. The Mets are set to move there in April.

Rob Bedelis, a mechanical engineer who helped build the Milwaukee Brewers’ Miller Park, said Citi Field shouldn’t be rusting a little over two years after the start of construction.

“It’s a sign of the quality of workmanship,” he said. “If I were a fan, I wouldn’t be too thrilled. This might be cosmetic and not structural, but fans are paying a lot of money for baseball tickets.”

The contractor, Hunt-Bovis, didn’t return phone messages.

A Met spokesman said, “We are aware of the rust [and] plan to take care of it. It is no concern.”

Some Met fans, however, weren’t too happy.

“That is disappointing,” said 25-year-old Jeffrey Ku of Flushing. “It is supposed to be a state-of-the-art stadium, and it has rust. They might as well have stuck with Shea Stadium.”